Orientation of tubular polypropylene film

ABSTRACT

A process for orienting tubular polypropylene film to minimize shrinkage and total haze of the oriented film comprising reducing the temperature drop and maintaining a low bubble hoop stress prior to quenching.

United States Patent Hovermale et al.

[451 Mar. 27, 1973 ORIENTATION OF TUBULAR POLYPROPYLENE FILM Inventors: Ralph Allen Hovermale, Clinton, Iowa; George Joseph Ostapchenko, Williamsville, N.Y.; Hung Han Yang, Clinton, Iowa Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

Filed: Feb. 10, 1971 Appl. No.: 114,355

Related U.S. Application Data Continuation of Ser. No. 765,177, Oct. 4, 1968, abandoned.

U.S. Cl. ..264/40, 264/89, 264/95, 264/210 R, 264/289 Int. Cl. ..B29c 17/07, B29d 7/24 Field of Search ..264/95, 210 R, 89, 40, 234,

[56] References Cited UNITED STATES PATENTS 3,499,064 3/1970 Tsuboshima et al ..264/95 3,231,642 1/1966 Goldman et a]. ..264/25 3,325,575 6/1967 Last I ..264/95 3,248,463 4/ 1966 Wiley et al. ..264/95 Primary Examiner--Robert F. White Assistant ExaminerJeffery R. Thurlow Attorney-Donald W. Huntley ABSTRACT A process for orienting tubular polypropylene film to minimize shrinkage and total haze of the oriented film comprising reducing the temperature drop and maintaining a low bubble hoop stress prior to quenching.

3 Claims, 3 Drawing Figures PATENTFDHAR27 197s SHEET 10F 2 FIG. 2

FIG.

INVENTORS ALLEN HOVERIALE A RNEY PATENTEDRARZYIHB SHEET 2 BF 2 GRADIENT l5G Y/ NAN. TENP.

LTENP. GRADIENT o 0 0 M W m a. use.

DISTANCE INVENTORS RALPH ALLEN HOVERNALE GEORGE JOSEPH OSTAPGHENNG P HUNG HAN YANG ATTORNEY ORIENTATION OF TUBULAR POLYPROPYLENE FILM This application is a continuation of Ser. No. 765,177 filed Oct. 4, 1968 and now abandoned.

BACKGROUND OF THE INVENTION Polypropylene film is frequently extruded in tubular form, and is biaxially oriented while still in a tubular configuration. Orientation is effected by simple longitudinal stretching of the tubular structure, combined with expanding the tube by internal pressure. This type of orientation can be carried out with apparatus of the general type described in Goldman et al., US. Pat. No. 3,231,642, hereby incorporated by reference. In such apparatus, the tubular structure is drawn past a heating element to raise the temperature to within the orientation temperature range of the film. Shortly after passing the heating element, upon reaching the orientation temperature, the tube is expanded by the air pressure inside the tube. Conventionally, a current of air is passed over the exterior surface of the tube to improve gage uniformity and to provide the required cooling curve stretch. At the tubes maximum expanded diameter the film is quenched by passing the expanded tube through a cooling sleeve.

While this process satisfactorily orients the film, several difficulties are encountered in its commercial application. For example, orientation often degrades the optical qualities of the film and yields a product which exhibits a substantial degree of heat shrinkage after orientation. Continual attempts have therefore been made to improve the clarity and reduce the shrinkage of polypropylene films prepared and oriented by a tubular process.

SUMMARY OF THE INVENTION The instant invention provides a process for the orientation of tubular polypropylene films which results in a minimal impairment of the optical properties of the film and a combined machine direction and transverse direction shrinkage of less than 25 percent. Specifically, the instant invention provides an improvement in the process for orienting tubular polypropylene film by heating the film to a temperature within the orientation temperature range and biaxially stretching the film by radial expansion and linear extension of the tube, and cooling at maximum expansion, the improvement comprising a. maintaining the temperature drop of the film to not more than C. from the initial radial expansion to the point at which the tube reaches its maximum expanded diameter and b. maintaining a bubble hoop stress no greater than 3,200 psi prior to cooling.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are schematic illustrations of two specific types of apparatus which can be used in the process of the instant invention.

FIG; 3 is an illustration of a representative temperature profile in the process of the instant invention along various points in the expanding tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It is known in the art that minimum shrinkage in a biaxially oriented polypropylene film can be approached by a simultaneous, uniform stretching of the film in mutually perpendicular directions under isothermal conditions. While attempts have been made to modify the process for orientation of a tubular polypropylene film structure to approximate simultaneous stretching along the machine direction and transverse direction axes, these conditions have been commercially impractical. In conventional tubular orientation processes, it has been found that the machine direction and transverse direction orientation is to some degree sequential, in that the desired degree of machine direction stretching is generally completed before one-half of the desired degree of transverse direction stretching has been accomplished.

The instant invention rests on the discovery of other factors which contribute to the shrinkage of oriented film. Specifically, it has been discovered that the shrinkage of the oriented film to a certain extent varies inversely with the temperature at which the film is oriented, and that the shrinkage of the oriented film increases with larger temperature drops during the orientation process. Still another factor that has been found to be critical in the shrinkage of the oriented film is that the final shrinkage varies directly with the bubble hoop stress during the orientation. By regulation of these additional factors, it has been found that the shrinkage of polypropylene film oriented in a tubular process can be maintained below 25 percent, a level heretofore unobtainable in a commercial tubular orientation process. The term shrinkage is used herein to mean the additive total of the percent shrinkages along both the machine and transverse axes of the film when exposed to a temperature of C. for 1 minute.

As indicated above, one of the critical factors in the attainment of minimal shrinkage of the oriented film is the maintenance of the temperature of the film during orientation within 15 of the original temperature to which the film was heated at the start of the orientation. This is basically contrary to the methods previously used in the art, in which the tubular film experienced a drop of greater than 20C. between the initial expansion and the point at which the film had attained its maximum expanded diameter, simply by atmospheric cooling. The temperature drop in the orienting film was even greater in those processes wherein cooling air currents were circulated around the exterior of the expanding film, the temperature drop being as much as 30 to 50C. According to the instant invention, the temperature of the film during this period is maintained within 15 of the temperature at which orientation is begun. This can be accomplished by any convenient heating apparatus. For example, a stationary heated element can be placed on the exterior of the expanded film. Such a heating element can be a radiant heater or can be internally heated by electrical coils or by the circulation of a heated fluid such as steam or oil. It has been found especially convenient to provide a coil in close proximity to the exterior of the tube having a fluid circulating therethrough at a temperature of about from 50 to C.

It is preferred that the temperature of the film be maintained above 130 in order to give a film that is heat stable for a broad range of commercial applications. Since the normal temperatures to which polypropylene is heated for orientation are about from 140 to 158C., the minimum temperature to which the orienting film should fall prior to quenching should be in the range of 130 to 143C.

The second critical element in the process of the instantinvention is the maintenance of the bubble hoop stress at a relatively low level prior to-quenching after stretching. This factor rests on the discovery that the shrinkage of the oriented film decreases with decreasing bubble hoop stress regardless of the orientation geometry. The bubble hoop stress is related to the air pressure within the expanding film as well as the diameter and thickness of the film according to the following formula:

where S hoop stress P bubble pressure d diameter at maximum expansion t thickness at maximum expansion The bubble hoop stress in the process of the instant invention, to achieve a minimal shrinkage in the oriented film must be maintained below 3,200 psi as determined by the above formula.

The instant invention will be better understood by reference to the Figures. FIG. 1 illustrates one particular apparatus which can be used in the orientation of tubular films according to the improved process of the instant invention. In that figure, the tubular film 1 is moved toward a guide ring 2, the ring serving to minimize sway of the tube. The tube is moved by means of tube advancer3 and a set of nip rolls 4 rotating at a rate that is at least 2 times the rate of the tube advancer. The film is advanced through an initial radiant heater 5 concentrically positioned around the periphery of the tubular film, and connected to a power source, not shown. The initial radiant heater 5 serves to heat the film to a temperature within about 70 percent of the lowest temperature of the orientation temperature range to just below the orientation temperature range of the thermoplastic polymer.

The tubular film 'is brought to a temperature within the orientation temperature range and at which the film expands by means of a pencil-type, internal, centrally located radiant heater 6 mounted on the internal guide ring 7. Heater 6 is connected to a power source by wires leading through the conduit 8 to a power source, not shown. The tubular film, immediately upon reaching the orientation temperature range, starts to expand due to pressure within it and to elongate due to the relative rates of the nip rolls 4 and the tube advancer 3. Air or other gaseous medium is admitted through inlet 11 and vented through outlet 12 to regulate the pressure within the tube.

The expanding tube, as it is removed from the immediate proximity of internal heater 6, is heated by external heating element 9, which maintains the film within the desired temperature range until it reaches its maximum expanded diameter. Upon reaching its maximum diameter, the expanded tube is passed through cooling ring 10. The cooling ring is generally maintained at a temperature of about from 5 to 10C. by circulation of cold water through the ring.

FIG. 2 illustrates another specific apparatus arrangement useful in the instant invention having an air cooling ring 13 inserted with the orifice of the ring directed at the plane of initial expansion. The cooling ring is regulated so as to improve gage uniformity as described in Goldman et al., US. Pat No. 3,231,642, while maintaining the temperature drop of the expanding film within the required limits.

FIG. 3 is a diagrammatic illustration of the temperature profile of the film during the orientation process. The upper figure schematically illustrates the film during its expansion, consecutively passing the internal heater 6 and external heating element 9 and the quench ring 10. The graph in the lower portion of the figure illustrates the temperature profile during the expansion. The vertical scale indicates the temperature of a representative tubular polypropylene film while the horizontal scale directly corresponds to the expanding film illustrated in the upper portion of the figure, so as to indicate the temperature of the film at the various points in its expansion. The temperature profile of a representative film oriented according to the process of the instant invention is indicated by a solid line in the lower portion of FIG. 3 while a representative temperature profile of films oriented according to processes previously used in the art is indicated by the broken line. As can be seen from the figure, the maximum temperature gradient between the initial heating at the beginning of the orientation process and the point at which the expanding tube reaches its maximum diameter is 15 in the process of the instant invention, while the temperature gradient exceeds 20 in the standard process.

The invention is further illustrated by the following specific examples.

EXAMPLE 1 A polypropylene film is extruded and stretch oriented on an apparatus of the type illustrated in FIG. 1. A polypropylene resin having a melt flow of 5.5 to 6.5 grams/ 10 minutes, a density of 0.905 grams/cc. and a crystalline melting point of C. is extruded into the form of a tube at the rate of 39 lbs/hr. at a temperature of C. The tube is extruded through an annular die having a 5 inch diameter. The film is quenched by passing the tube through a mandrel maintained at a temperature of 5C. After the initial quenching, the tube has a film thickness of 14 mils and a temperature of 30C. The tube is reheated by an internal heater to a temperature of 148C. and is expanded to stretch orient the film. The expanding tube is encircled by a heated ring having a diameter of 25 inches. The heated ring extends along the tube for 21% inches and is maintained at a temperature of 110C. The tube is expanded to a diameter of 25 inches by a bubble hoop stress of 2,550 psi. The temperature drop between the initial heating and the point to which the tube reaches its maximum diameter is 9C. At the maximum diameter of the tube, the film has a thickness of 0.57 mils and has been stretched 4.76 times in the machine direction and 5 times in the transverse direction. Theexpanded tube is passed through a quench ring having a diameter of 25 inches and a height of 6 inches. The quench ring is maintained at a temperature of5C. and the expanded tube, upon leaving the quench ring, has a temperature of C.

combined machine direction and transverse direction shrinkages of the resulting film exceed the desired 25 percent.

We claim:

The cooled oriented film is then tested for shrinkage 5 l. in the process for orienting a tubular sheet of by subjecting it to a temperature of 130C. for 1 polypropylene film by heating the film to a temperature minute. The film exhibits a machine direction shrinkwithin the orientation temperature range and biaxially age of 8.4 and a transverse direction shrinkage of 10.7 tr tching the film by radial expansion and linear extenfor a total shrinkage of 19.1 percent. sion of the tube, and cooling by quenching at maximum The total haze for the oriented film is 2.75 percent. 10 expansion, the improvement for P di g a film ha g a combined machine direction and transverse direction EXAMPLES shrinkage after orientation of less than 25 percent and The procedure of Example 1 is repeated using a total haze of less than about 5 percent which comprises pzlypmpylene (T under P condltlons' a. forcmg a gas upon the exterior of the tubular sheet T e process varlab es and the resultmg product characwhile expanding terlstics are summa rlzed 1n Table 1. lixamples 2 and 12 Subsequently passing the tubular sheet in proximi illustrate a conventional process having a film temperaty to a heat Source having a temperature of at least ture drop and/or b ubble hoop stress outside of the 50 as to maintain the temperature drop of reqmremems ofthemstantmvemlon' the film to not more than 15C. from the initial radial ex ansion to the oint at which the tube EXAMPLES 13-19 P reaches 1ts maximum expanded dlameter and The procedure of Example 1 is repeated using a c. maintaining a bubble hoop stress no greater than polypropylene commercially available from the Her- 3,200 psi prior to cooling. cules Powder Company as Profax TB333. The mol- 2. A process of claim 1 wherein the temperature of ten polypropylene tube is extruded at the rate of 500 the expanding film is maintained above 130C. lbs./hr. The process variables and the resulting product 3. A process of claim 1 wherein the temperature characteristics are summarized below in Table 2. drop of the film is regulated by a heating means on the TABLE 1 7 Film Heated Film Shrinkage percent thlekring MD x TD Bubble Hoop temp. at 130 C./1 min.

ness, temp, stretch pressure stress, (1r mil C. ratio in H2O p.s1 C MD TD Total 0.5 55 5x5 4 3,600 15 s 2 17.2 25.4 0.5 75 5 x 5 3.5 3,150 12 0 13 22 0.56 00 5x 5 3 2,400 4 3 7.5 13.5 0.55 110 5x 5 2.5 2,010 6 6.5 0.0 15.5 0.56 115 5x 5 2 1,500 4 5.7 8.2 13.0 0.6 61 6x 6 7.0 2,530 0 6.7 12.2 18.9 0.6 71 6x6 5.0 1,880 1 5.0 5.0 11.0 0.6 71 5x6 3.0 1,125 6 2.7 3.7 6.4 0.50 00 5 x 5 3.25 2, 430 10 0.2 10.8 20.0 0.50 110 5x5 3.25 2,480 10 8.4 10.7 10.1 0.5 5x5 4.0 3,600 20 10 20 TABLE 2 Example Orientation conditions:

MD stretch ratio 5.0 5. 0 5.0 5.0 5.0 (1.0 5. 0 T1) stretch ratio 5. 0 5. 0 5. 0 5. 0 .5. 0 5. 0 5. 0 Bubble pressure in 1120 1. 0 1.3 1.0 0.05 0. 1.0 1.0 lIoop stress, p.s.i 3,000 2, 300 1,010 1,370 1,700 2, 0.15 4,120 l1eatetlring, (1 I10 "10 70 70 Air ring:

F.p.m 1,200 1,000 000 000 0 1,300 C 70 50 50 50 100 73 External heater, C 144 144 144 1114 144 144 144 TI) initiation tcmp., o 143 14:11 140 14x 14x 10* Film temperature gradient, 0.. 20 10 11 8 '1 7 l6 Quench ring, C 18 18 18 18 8 2% Cast tube: Avg. gage, 111ils 18 18 18 1X 1% 7 Oriented film:

Avg. page, mils 0. 60 0. 73 0. 03 0. 65 0.05 0. 02 0. 00 1107.0, percent: Total 2. 1 3.0 3.3 J. 4 2.3 1.7 Shrinkage percent at 130 M1) 0. ii a. 7 3.7 0.2 7.7 3.2 12.3 To. 20.0 17.7 12.7 11.3 0.3 0.0 20.2

It may be noted that in those examples in which the film temperature drop (gradient) and/or the bubble hoop stress exceed the limitations of the process of the instant invention, namely, Examples 13, 14, and 19, the 65 exterior of the tube having a. fluid circulating therethrough at a temperature of about from 50 to C. 

2. A process of claim 1 wherein the temperature of the expanding film is maintained above 130*C.
 3. A process of claim 1 wherein the temperature drop of the film is regulated by a heating means on the exterior of the tube having a fluid circulating therethrough at a temperature of about from 50* to 155*C. 